1. Field of the Invention
The present invention relates to a method of readjusting the images from a synthetic-antenna sonar, which is formed by the shifting of a linear antenna of transducers with length L. All the transducers are active in reception and some of them are also active in transmission. The transmitters may also not form part of the antenna.
2. Discussion of the Background
Synthetic antennae are known in which a virtual antenna is formed from the different spatial positions of a physical antenna.
When the nominal motion (without movement error) of the carrying vehicle is collinear with the physical antenna, a synthetic linear antenna is obtained. One known embodiment relates to lateral sonars for seabed imaging, for which the synthetic antenna makes it possible considerably to enhance the lateral resolution.
When the nominal motion of the carrying vehicle is perpendicular to the physical antenna, a bidimensional synthetic antenna is obtained, often having gaps. One embodiment relates to frontal minehunting sonars. The synthetic antenna then allows supplementary antenna gain with respect to the physical antenna in order to enhance the performance in terms of detection and classification. This enhancement results from the fact that the coherence, from one recurrence to the next, of the echo from a mine is generally higher than that of a bottom echo, which is itself higher than that of the noise.
The focusing of a synthetic antenna at a given point F, however, requires compensation for the differences in outwards and return path between F and the physical sonar for all the K consecutive recurrences which it is desired to integrate. However, an error in the movement of the carrying vehicle of .lambda./4 between two consecutive recurrences, i.e. less than a millimeter in the case of the wavelengths usual in mine-classifying sonar .lambda.=3.75 mm for f.sub.o =400 kHz), is sufficient to sum the echoes originating from F in anti-phase rather than in phase and thus to destroy the synthetic antenna. The instrumentation, which moreover is complex and expensive, does not have the required precision. Moreover, when movement errors are present, the corrections to be made to the path differences depend on the projection of these errors in the radial direction of F, which depends on the relief of the bottom. However, this remains unknown when the physical antenna is, as assumed here, a linear antenna which exhibits a conical ambiguity. Auto-focusing is then one particularly attractive solution. It makes it possible to get round the difficulties relating to the separate measurement of the movement of the carrying vehicle, of the relief of the bottom and of the profile of instantaneous ultrasonic velocity. In place of that, the combinations of these quantities are estimated, namely the differences in path length, which are required for the problem posed.
A French patent, No. 94 11464 is known, filed by ICPI of Lyons, with J. Chatillon and J. Magand [1] as inventors, which aims to define such an auto-focusing method. In contrast to the present invention, this method does not take advantage of the multi-sensor structure of the physical antenna. It assumes, moreover, that the scene imaged can be segmented into regions including a single diffuser. It is therefore inapplicable in the case, important in minehunting, where the scene imaged includes only one anechoic mine against a bottom of uniform reverberation, for example a homogeneous bottom of mud or fine sand (in this case the mine is detected by the shadow it casts). This is because even the smallest region which can be segmented in the imaged scene, which is nothing more than the resolution cell of the sonar, still contains a continuous infinity of diffusers, according to the conventionally accepted model for describing a bottom of uniform reverberation.
A U.S. Pat. No. 4,224,036, granted on Jan. 6, 1981 to Westinghouse Electric Corp. is also known, with R. S. Raven as inventor, in which is described a method of auto-focusing for a synthetic-antenna lateral sonar, which can operate even against a bottom of uniform reverberation. This method makes it possible to integrate two consecutive recurrences, the extension to the case of K recurrences being done step by step. It uses at least one transducer being shifted in reception mode between these two recurrences. The transducer is shifted relative to the physical antenna in such a way that its center of phase, defined as the place which is the geometric middle of the center of the transmission antenna and of the transducer, remains nominally fixed relative to the water. It is seen in FIG. 1 that it has to be shifted by .DELTA.R=2 vT in the direction opposite the movement of the sonar, where vT is the nominal motion of the carrying vehicle between two recurrences. Such a motion is possible on condition that L&gt;2 vT, that is to say that the synthetic antenna is oversampled spatially.
Under these nominal conditions, the signals at recurrences No. 1 and No. 2 from the shifted transducer are identical. In the presence of errors, a non-zero phase shift between the two complex samples relative to the echoes from the same range cell of the bottom would be observed, according to the method of this patent [2]. This phase shift would then furnish the phase correction to be applied, range cell by range cell, to the signals from all the transducers of recurrence No. 2 so as to integrate them with the signals of recurrence No. 1. It should be noted, however, that the application of the method requires preliminary readjustment, by another method not specified by the patent [2], of the paired complex samples the precision of which should be a fraction of 1/B where B is the passband of the sonar.
This method [2] also describes an extension to the case of the simultaneous use of several shifted transducers. This is because, when the synthetic antenna is sufficiently oversampled spatially, it is possible to shift at least two transducers in reception mode, to estimate at least two phase shifts according to the preceding method, and to extrapolate these multiple estimates to a phase-shifting law varying linearly along the physical antenna. This compensation law is then applied to the signals from the transducers at recurrence No. 2 in such a way as to achieve, in addition to a constant phase correction, electronic aiming-off of the physical antenna at recurrence No. 2.
Finally the method [2] also makes mention of the possibility of shifting the transmitter by 2 vT, which allows the simultaneous use of all the transducers of the physical antenna, resulting in a gain in precision. However, a problem is posed, described by the inventor of [2], when it is sought continuously to readjust a series of consecutive recurrences. Hence the backward movement of the transmitter by 2 vT between recurrences No. 1 and No. 2 makes it possible to readjust this pair of recurrences but not the recurrences No. 2 and No. 3, since the transmitter has arrived at the end stops of the physical antenna. It is only possible, in fact, to readjust every other pair of recurrences.
Another problem, not described by the inventor of [2], is that the shift on transmission cancels the nominal movement of the center of phase of the physical sonar in such a way that there is no longer spatial diversity, and thus no gain in resolution, for the coherent integration of the two recurrences thus readjusted. Auto-focusing is thus obtained at the expense of the performance of the synthetic antenna which is clearly not desirable.
The modes of shifting on transmission which are proposed in the U.S. patent application No. 9,510,953 filed on Sep. 19, 1995 in the name of Thomson-CSF [3] make it possible to overcome these two limitations. One of these modes uses two alternate auxiliary transmissions (alternating at each recurrence), which are carried out with pairwise codes distinct from each other, and distinct from the main transmission code, carried out with a fixed transmitter. The three codes are, for example, in distinct sub-bands, which makes it possible to separate them by filtering on reception. Thus, for every pair of recurrences of a continuous series, there always exists one of the two auxiliary sub-bands in which it is possible to shift on transmission in the direction opposite to the movement of the carrying vehicle. With these shifting movements being in sub-bands unconnected to the main transmission, moreover, they have no effect on the imaging. All types of spectral multiplexing are possible, for example those making it possible, by beating, to form two auxiliary codes, spectrally unconnected but with the same central frequency as the main transmission.
However, certain fundamental limitations of the method [2] persist. The fact that the shift on reception or on transmission is determined by the nominal movement vT of the carrying vehicle, without taking account of movement errors, is a source of inaccuracy. Moreover, the method [2] assumes that it is possible to apply the same phase corrections for all the diffusers F of the same range cell. This condition of isotropy of phase errors is known in optics by the name of isoplanetism. The method is applicable only in the case where the transmission sector in its entirety can be regarded as an isoplanetic sector which is a limitation, in practice, as the following example shows.
In an Oxyz reference system as in FIG. 2, the seabed is above the plane Oxyz and the sonar is being nominally shifted along Ox. It is assumed that the only parasitic effect is a heave effect of amplitude P along Oz between two consecutive recurrences. This heave means that the positions C.sub.1 and C.sub.2 of the centers of phase of a shifted transducer are no longer coincident. The co-ordinates along Ox and Oy are equal, by construction, but it is otherwise along Oz, where the parasitic movement leads to a vertical separation of P between the two points, illustrated in FIG. 2. It can then clearly be seen from the Figure that the phase error is not the same for the diffusers F.sub.1 and F.sub.2 of the range cell. This is because C.sub.1 C.sub.2 can be seen as a synthetic interferometric base along Oz, with resolution of .lambda./2P, so that the phase error can be assumed to be constant only if the variation in azimuth, relative to the axis of the interferometric base, of the echoes from the range cell remains of the order of .lambda./16P. For P=10 cm and .lambda.=3.75 mm, the maximum allowable variation is about 0.1.degree.. It is therefore sufficient, at a range of 200 m and for the grazing incidences usual in minehunting, to have altitude variations of the order of 50 cm in the range cell in order to reach the limits of the method. For a transmission sector of 5.degree., the range cell situated at 200 m extends along Ox over more than 20 m. It is therefore sufficient for a bottom which is flat but inclined by 10% relative to Oxy in order to have altitude variations of more than 2 m. Extended dunes, or ridges, the amplitude of which lies between 40 cm and 1 m, are also frequent on the seabed. This analysis is, obviously, not limited to the case of heave alone and range cells at the limit of range. In mid-range or at the start of range, combinations of parasitic yaw and heave produce identical effects. The same goes for attitude errors. Hence, for L.sub.r =4 m and pitching of only 1.5.degree., P=10 cm is already reached.
In the presence of such angular variations, the constant phase shift estimated by method [2] loses its meaning.
In order to attempt to increase the robustness of method [2], patent application [3] proposes forming auxiliary transmissions with finer beams than that of the main transmission. In one embodiment, the auxiliary beams are of 2.degree. whereas the main beam is of 8.degree.. However, in order to have all the gain in resolution of the synthetic antenna, the phase errors have to be estimated over the entire main transmission sector of 8.degree. and not only over a sub-sector of 2.degree.. It would be necessary at least to cover the main transmission sector with a plurality of auxiliary transmissions with beams as fine as possible, the transmissions being in unconnected sub-bands according to the known technique of channel formation on transmission.
This analysis reveals the fundamental limitation of the system of shifted transducers. They do not have sufficient angular resolution to estimate the angular variations of the phase errors which are sought.
The problem posed can be formulated as follows. Let F be a point on the bottom defined by the pair of sonar co-ordinates (u, .tau.) relative to the recurrence 1. Let u=cos .theta., where .theta. is the azimuth of F relative to the physical antenna and .tau. the round-trip propagation delay, as in FIG. 3. The co-ordinates of F at recurrence 2 are sought, in order to form the synthetic antenna. These can be expressed as a function of the co-ordinates (u, .tau.) at recurrence 1 in the form:
u+.DELTA.u(u, .tau.),.tau.+.DELTA..tau.(u, .tau.) (1)
The problem posed is that of estimating the shifts .DELTA.u and .DELTA..tau. of (1), for all (u, .tau.) of physical image No. 1, from the sonar signals alone. It is posed in an identical way for all the synthetic modes implemented with a linear antenna (lateral sonar, frontal sonar, etc.) and especially for the modes described above.
It is also posed in the same way for all the applications aiming precisely to pair two images of the same region of the bottom for the purposes of navigation readjustment or objective designation. Numerous image processing techniques are known for doing this, which are used for radar or optical images, based on extracting significant points from images No. 1 and No. 2 and pairing them. However, these techniques do not operate against a flat bottom with uniform reverberation, which is a limitation in sonar where the density of the noteworthy points (often of human origin) is much lower than in radar. The image processing techniques aiming to carry out intercorrelation of the two images do not have this limitation but come up against other difficulties. On the one hand, the shifts .DELTA.u and .DELTA..tau. are not constant over the image, and, on the other hand, the coefficient of correlation of these images is low, close to 1-2 vT/L for a lateral sonar against a bottom of uniform reverberation.
There is also known, from a U.S. Pat. No. 4,635,240 granted to the Westinghouse company on Jan. 6, 1987, with G. Geohegan and C. W. Allen [4] as inventors, a navigation system using a frontal sonar to estimate the amplitude and the direction of the horizontal shifting of the carrying vehicle between two recurrences. This method makes use of the intercorrelation of channels formed with the physical antenna assuming that the antenna is stabilized in attitude, the vertical movement of the carrying vehicle is known, as well as the depth of water. The bottom is, moreover, assumed to be flat. Unlike the method according to the invention, this method requires the aid of navigation sensors and makes assumptions as to the relief, which makes it too imprecise for the applications envisaged here. Moreover, it does not deal with the case of lateral sonar.